The present invention is concerned with the detection of fault currents in electricity power lines. It is a common practice for electrical power systems to be equipped with means for detecting excessive currents so that faulty parts of the power system can be automatically isolated. Such devices may be over current relays or sectionalisers (such as that disclosed in our U.S. Pat. No. 4,553,188). Excessive current can be detected by any means that recognizes that the magnitude of the current in the power system has exceeded a given threshold value.
A problem with existing fault and over current detection systems is that they are prone to be actuated in response to short term current excursions which may occur in a normal healthy powre system. A particular problem is that resulting from transformer magnetising inrush currents which occur when a section of the distribution system is energized, resulting in the energising of one or more distribution transformers. Former current detection devices have typically been set with their threshold values sufficiently high to ensure that transformer magnetising inrush currents do not actuate them. Such threshold values may then be considerably higher than the ideal threshold value which would be chosen to detect all fault currents if transformer magnetising inrush currents were not a problem. As a result the overall protection of the power system is sometimes not as good as would be desirable.